1. Field of the Invention
The present invention relates to a photoelectric converting device and an image processing apparatus utilizing the same, and more particularly to a photoelectric converting device of high sensitivity and high response capable of high-speed image reading, and an image processing apparatus utilizing said photoelectric converting device.
2. Related Background Art
In image information systems, optical communication and other industrial and consumer fields utilizing light as the medium of information signals, the semiconductive photosensor for converting optical signals into electrical signals is one of the most important and most basic components, and is already commercialized in various structures.
Such a photosensor is generally required to have a high signal-to-noise (S/N) ratio, a high sensitivity for image reading and a high response speed, during the photoelectric conversion. Further, for applications as the input device for a high-speed facsimile, an image scanner, a copying machine or other image processing equipment, said photosensor is desired in the contact configuration for facilitating compactization of such equipment, and the formation of a large-area element array is required. On the other hand, in the area sensors employed, for example, in the industrial or consumer video cameras, the pixel area has to be formed as large as possible, as the output of such photosensor generally becomes smaller as the density of pixels becomes higher. Based on these situations, technical development for the photoelectric converting device is being directed to signal processing circuits and the photosensor in a superposed structure, thereby effectively utilizing the surface area.
For meeting these requirements for the photoelectric converting device, a photodiode having a PIN structure, based on amorphous silicon is considered useful.
However, in the simplest structure of such a PIN photodiode, all the layers are formed with amorphous silicon. It is difficult to achieve, at the practical level, (1) a reduction of dark current by blocking minority carriers, which is an important function of the P- or N-doped layer, and (2) a low afterimage characteristic, at the same time.
This phenomenon will be explained in more detail with reference to FIG. 1, which is a schematic cross-sectional view of a photoelectric converting device. In FIG. 1 there are shown a glass substrate 151; an electrode 152 for example of chromium; an amorphous N-silicon carbide layer 153; an amorphous I-silicon layer 154; an amorphous P-silicon carbide layer 155; and a transparent electrode 156. In the device illustrated in FIG. 1, the minority carriers injected from the electrodes can be effectively blocked, and the dark current can thus be reduced, by forming the P- and N-semiconductor layers with materials of wider energy gap than that of the material constituting the I-semiconductor layer. However, an energy band discontinuity and an interface trap level resulting from a junction of different materials are formed at the interface between the P-and I-semiconductor layers, and the resulting carrier trapping at said interface creates the afterimage phenomenon.
Therefore, said interface is preferably free from the energy band discontinuity and the trap level for reducing the afterimage phenomenon. For this reason, there have been attempts to form the P-and N-semiconductor layers with amorphous silicon in the same manner as the I-semiconductor layer. Such configuration eliminates the energy band discontinuity and the trap level at said interface, but results in an increased injected current, since the doping efficiency cannot be effectively increased, thus leading to an increase in the dark current.